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EXAMPLE
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a(3) = 4 since there are only 4 numbers in English with 3 letters: one, two, six, ten.
a(4) = 4 since there are only 4 numbers in English with 4 letters: zero, four, five, nine.
a(10) = 35 (24, 25, 29, 34, 35, 39, 43, 47, 48, 53, 57, 58, 63, 67, 68, 71, 72, 76, 84, 85, 89, 94, 95, 99, 100, 200, 600, 1000000, 2000000, 6000000, 10000000, 1000000000, 2000000000, 6000000000, 10000000000)
a(11) = 32: [2389][378],7[459], [459]00, [126]000, 10000, [459]000000, [459]000000000, [126]000000000000, 10 trillion.
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